In the design of gas turbine engines, it is important to not only construct a combustion system that performs efficiently, but also to construct one that minimizes undesirable emissions such as carbon monoxide (CO) and oxides of nitrogen (NOx) as well. A well-known method of minimizing such emissions involves injecting fluid coolants, such as water, into the combustion chamber in order to reduce the flame temperature therein.
In the known water injection systems, water is typically injected from a position immediately radially outwardly of the fuel source, i.e., atomized water is fed through the swirl vanes of the fuel injector tip. However, this method causes extreme thermogradients in the surrounding swirl cup, trumpet, dome plate, and other combustion chamber components as a result of the direct impingement of water droplets on the hot component parts. The resultant thermal stresses create cracks and greatly reduce the useful life of the components, necessitating increased maintenance, repair and inspection frequency.
A prior attempted solution to this problem involved constructing the affected components out of metal materials that are less prone to cracking and/or insulating the metals with thermal barrier ceramic coatings. While both of the above approaches have had some success in extending the useful lives of the components, cracking nonetheless occurs.
Another approach, described in U.S. Pat. No. 5,274,995, employs a combustor dome assembly having a swirler venturi and an auxiliary wall concentric with the swirler venturi to provide an annular passage for channeling a high velocity non-swirling annular air jet surrounding the swirl air injected by the swirler surrounding the fuel nozzle. This arrangement facilitates atomization of a film of water flowing along an inner surface of the venturi and out of the downstream end. However, among the problems associated with such an approach is the requirement of a relatively large prefilming surface located coaxially with and outboard of the fuel nozzles and the need for a complex injector capable of handling both fuel and water. Additionally, the approach disclosed in the '995 patent is similar to the myriad traditional approaches, in that the point of water introduction into the combustion chamber is proximal the swirl cup, trumpet, and dome plate thus perpetuating the likelihood that at least some water droplets will impact these components.
Additionally, it is often desirable to augment turbine engine thrust during take-offs and during various emergency conditions. Such thrust augmentation is often accomplished by introducing a fluid into the combustion chamber, thereby increasing the mass flow through the turbine inlet. As discussed above, fluid coolants such as water, whether injected for emissions reduction purposes or for thrust augmentation, are ideally introduced well downstream of sensitive fuel nozzle components. One approach to thrust augmentation, described in U.S. Pat. No. 2,847,825, involves injecting coolant into a combustion chamber through large holes disposed along a can-type combustion liner. Among the problems associated with the approach described in the '825 patent is that no provision is made therein to properly atomize the coolant prior to its introduction into the combustion chamber. Consequently, the approach described in the '825 patent is likely to cause water droplet impingement upon hot combustion chamber and/or turbine components and the creation of disproportionately cool regions with the chamber, thereby reducing engine efficiency.
Accordingly, a need exists for an improved apparatus and method for injecting fluid coolants into a gas turbine combustion system that minimizes damaging thermogradients in sensitive combustor components caused by coolant impingement thereupon.